1. Field of the Invention
The present invention relates generally to registration references for the print head of a high resolution laser or ink-jet printer or a plotter, and in particular to a true-dimension optical encoder strip for a wide-format printer or plotter.
2. Description of the Prior Art
Laser and inkjet printers and plotters utilizing a wide variety of technologies are well known to the art. The term "printers" is used herein generically referring to color and monochromatic (black-and-white) laser printers, inkjet printers, and plotters, unless a particular distinction between these types of devices is specifically called for or noted. In general, many printers and plotters operate using substantially similar or interchangeable technology and components, but are utilized in different applications. Those of skill in the art readily appreciate these distinctions or limitations, and the relative advantages or disadvantages of the corresponding technologies.
Many commercial and personal printers have resolutions of 300 dots per inch (dpi) or greater, with 600 dpi and 1200 dpi becoming standard within recent years. Resolutions of greater than 2000 dpi can be achieved on some high end personal printers, and are conventional for professional printers, typesetting machines, and photoduplicating or photolithography machines.
The basic dot resolution of some printers can be further enhanced electronically by altering the size or shape of the individual dots, or interpolating between standard dot placements to achieve a finer spacing. This technique works effectively to smooth printed images and type faces because few printed elements are composed of single dots--or are spaced apart single dot widths--and the position of the terminal dots along a given line of dots each having predetermined width can be shifted linearly along that line relative to the adjacent dots to thereby overlap the adjacent dots and extend the line less than a full dot width. This electronic enhancement relies on filtering the raster elements of an image to determine better smooth line approximations given the available dot width and interpolation capabilities, and physically on the capacity of the device to accurately position the print head at less than dot-width intervals.
It is readily appreciated that such printers require great precision and uniformity in the ability to repeatably position the print head. Variations in this precision result in compression or expansion along individual lines of an image, or in elements which lack clarity or definition at the desired dot resolution. Variations between lines will result in abnormally dithered or skewed portions of images, or other irregularities in print quality or clarity. In many graphic (raster) images for personal or even professional use, these minor variations will not be readily detectable by normal visual inspection in most applications unless a particular screen pattern or color separation is involved which produces a cascade effect and creates visible distortions throughout larger portions of the total image. By comparison, this lack of precision cannot be tolerated for computer-aided design (CAD) applications. In the case of high resolution or enhanced resolution printers for professional applications, precise print head placement is required to achieve the expected dot resolution of the device over the entire width of the image. Because high resolution images in large formats can be very expensive and slow to produce, plotters are more frequently utilized in applications where a large format image is created (often composed of significant "white space"), but exact accuracy is expected in line weights and the spacing between individual lines, the curvature or length of lines, and the density of image elements.
As such, providing an accurate linear reference to uniformly and repeatably determine the registration or placement of a print head is indispensable for printers and plotters. Many such devices rely on encoder strips which ideally have a multiplicity of discrete markings or "ticks" similar to a rule, equally spaced from one another but without corresponding dimensional references such as inches or points relative to the terminal ends of the encoder strip. A sensor such as an optical emitter/detector is mounted on or near the print head or carriage, and produces a digital or analog signal pulse as the sensor passes and detects each marking. A RISC chip counts those signal pulses and calculates the position of the print head relative to one of the terminal ends of the encoder strip, or to the last reference position of the print head. Given the linear speed of the print head as it traverses the encoder strip--which may be predetermined or calculated using the markings counted during an elapsed time interval--the position of the print head between the confronting edges of two markings (or between opposing edges of one marking) may be readily approximated by linear interpolation as a function of time (assuming uniform spacing between the markings, uniform marking widths, and uniform print head speed).
However, in practice the uniformity or precision in the spacing and weight of markings on an encoder strip is very much less than ideal. This is due primarily to limitations in the fabrication processes which result in inaccurate registration references, and secondarily to the inability to control the affect ambient conditions have on the encoder strips when placed in their operating environment.
One known process to fabricate an encoder strip is metal surface etching. This process is limited to producing metal encoder strips (or strips of materials amenable to similar chemical or lithographic etching processes), and produces several drawbacks. Materials may be selected which have dimensional stability along the linear (longitudinal) direction, although materials with higher thermal stability are conventionally more expensive, particularly in lengths suitable for wide-format printers. Obtaining the base material in a form having uniform thickness and suitable surface smoothness is important. Protection against print-head contact is important, since etching produces a non-uniform or rough surface that could be highly detrimental to a print head if contact occurs. However, maintaining a minimal spacing between the print head and the encoder strip to maximize accuracy and responsiveness is also desired.
Encoder strips fabricated from a polymer sheet or film such as Mylar.RTM. are also known. The markings on these polymer film encoder strips may be imprinted in a variety of ways, however the ultimate accuracy of the encoder strip is limited by the precision of the imprinting process or apparatus. Very high resolutions for imprinting encoder strips could be achieved using a device such as a laser photoplotter designed for electronic tooling, printed circuit board (PCB) fabrication, or wafer photoetching processes. However, these devices lack sufficient size to imprint encoder strips for wide-format printers, and are expensive. Larger photoplotters such as those used in reprographics lack sufficient resolution (or default to a lower dimensional tolerance) to themselves achieve the desired accuracy in imprinting markings on an encoder strip, and these devices are traditionally used by individuals whose focus is the production of large scale graphic images where minor variations in raster precision is visually undetectable and therefore ignored or discounted.
In addition, a fundamental flaw has long existed in the basic design of many encoder strips used for wide format printers--and particularly to polymer film encoder strips--which makes fabricating an accurate encoder strip far more difficult. This deficiency is more the result of reliance by those of skill in the art on traditional "lines per inch" standards for calculating and controlling image resolution than an inherent defect in manufacturing equipment. **For example, one inch (1") of encoder strip imprinted for 300 dpi basic (physical) resolution would have an alternating pattern of 150 lines and 150 intervening spaces. However, each line and each space would be one three-hundredths of an inch (1/300") in width. Converting this to decimal form, each line (or space) would have a width of 0.00333333. . . inches, wherein the row of threes in the decimal would repeat infinitely. For suitable precision, the encoder strip would need to be imprinted using a device that provided accuracy to six decimal places, whereas most available devices default to only four or less decimal places of accuracy.
The industry has attempted to address this inherent deficiency in several different ways. One method is to use a high resolution imaging device (such as a photoplotter designed for electronic tooling as described above) to generate a master imprinted on glass (or another permanent material), and using a contact photoprinting process to reproduce encoder strips from that master. This is a relatively slow process, and care must be taken to prevent dust or other contaminants from affecting the contact print. The conventional process of contact printing from a master can lead to loss in image quality, which adversely affects the accuracy or precision of the encoder strip. For wide format encoder strips, the equipment for and corresponding complexity of producing the master can increase the ultimate cost of the encoder strips, and it is necessary to produce a unique master for each version of an encoder strip.
Another widespread method is to imprint markings having only thirty-three thousandths of an inch (0.0033") width and spacing, rounded down from the corresponding infinite decimal. The result is 150 lines and 150 spaces which extend along a total distance of 0.99"for each inch of encoder strip--or 99% of the total length of the encoder strip--for a 1% initial error factor overall. The encoder strip is then mounted by stretching the material to its full 100% length and pinning the opposing ends in place.
It will be readily apparent to those skilled in the art that the polymer strip will not stretch uniformly, thus leading to localized distortions in registration. Furthermore, stretching the encoder strip makes it more susceptible to further stretching, distortion, damage, and deterioration. Since the encoder strip is mounted at opposing ends, the intermediate portion of the strip can be displaced and damaged more easily, and will deform or stretch due to its own weight as well as environmental factors such as humidity. This can result in contact between the print head and encoder strip which may damage one or both, or slow the print head due to friction. Some distortion can be corrected electronically by mapping the distortion patterns in the stretched and mounted encoder strip to a calibration table, and then adjusting the print head controller and printed dot pattern based upon that table. This process would require embedded software to perform the mapping and correction functions, and would significantly impede optimal printing rates. Subsequent stretching or distortion of the encoder strip would require periodic calibration.
Alternately, for a printer being used exclusively for raster or graphic images, the encoder strip could be mounted without stretching, resulting in images printed at 99% of their true widthwise dimension. While this may not be noticeable in many applications, it is not suitable for CAD and other precision applications--and renders the output of two dissimilar printers inconsistent with one another--particularly as the printer increases to wide-format images on the order of 45" to 90" in width (where a 1% error equates with a 0.45" to 0.9" differential between spaced elements).
Another method utilized to correct for the inherent limitation in imprinting resolution is to round the line width and spacing upward rather than downward. Using a sixty-seven thousandths inch (0.0067") combined line width and space rounded up from the corresponding infinite decimal, 150 lines and 150 spaces extend along a total distance of 1.005" for each inch of encoder strip--or 1.005% of the total length of the encoder strip--for a 0.5% initial error factor overall. While this error is less relative to rounding down (assuming a combined spacing of 0.0067" can be achieved while maintaining tolerances), the error must either be incorporated into the printed image or corrected in some manner.
One option is to discard a predetermined number of markings and spaces from one of the terminal ends of the encoder strip. For example, in a 46" wide format, the 0.5% rounding error results in an additional 34.5 lines (45".times.150 lines/in..times.0.005) lines. Thirty-four lines and an additional space can be discarded from one terminal end of the encoder strip. Another option is to imprint less than all of the full markings, or a partial line or space (or both) per unit of distance. For example, imprinting 149.5 markings per inch by reducing the width of one line and one space by one half reduces the error to 0.17% (per unit distance or total error). The effect is to build a small error into each unit distance (i.e., 0.5 line width per inch). In either case, either the total image width or discrete rows in the image (or both) will be distorted or incorrect, and the ability to perform such an adjustment is dependent on the tolerances and capabilities of the imprinting apparatus.